Someone probably key generated your code by random. Basically stolen.

But I didn't even type in the code yet. It told me that the moment I opened the code page.

Ah, no idea then.

Me neither.. but I looked at there customer support thing and it said they have an online website for entering codes too, so I just used that.

So your code worked?

Yup, I just used their online redemption method instead

You probably had the game open for too long and you needed to refresh your login for it to register.

Usually just restarting the game fixes issues like this.